In the assembly of semiconductor devices, many electrical interconnections formed on the integrated circuits are made with thermosonic bonding. It is known in the art that the thermosonic bonding process used in wire bonding of electronic devices utilizes an ultrasonic transducer for providing ultrasonic vibrational energy to fine wires of electrically conductive bonding materials, such as aluminum, copper or gold wires, thereby bonding them onto the bond pads of a die or a substrate by mutual friction between the surfaces.
FIG. 1 is a sectional side view of a transducer 100 of the prior art. The transducer 100 comprises a horn 102 having a main body for transmission of ultrasonic energy including a solid cylindrical portion 104. An ultrasonic generator 112, which comprises a piezoelectric motor, is attached to one end of the horn 102 through a threaded hole formed at one end of the solid cylindrical portion 104. The ultrasonic generator 112 will provide the transducer 100 with ultrasonic bonding energy when it is energized during the bonding process.
The horn 102 further comprises a frusto-conical portion 106 that extends from the solid cylindrical portion 104 and converges towards a distal tip 108 at the end of the horn 102. A bonding tool, such as a capillary 110, is attached to a hole formed near to the tip 108, and the capillary 110 is arranged orthogonally to the longitudinal axis of the horn 102. The capillary 110 has a hole extending centrally through its body for receiving fine bonding wires used during wire bonding.
When ultrasonic energy is supplied, it will be transmitted from the ultrasonic generator 112 to the horn 102, through the capillary 110, and eventually to the tip of the capillary 110. The tip of the capillary 110 will thereby oscillate in accordance with a characteristic frequency and corresponding amplitude of vibration. Hence, the wire at the tip of the capillary 110 may be ultrasonically welded onto a bond pad on a die or a substrate when the wire is pressed against the bond pad by the capillary 110.
The ultrasonic energy generated by the ultrasonic generator 112 will excite the transducer 100 such that a varying amplitude of ultrasonic vibration along the longitudinal axis of the transducer 100 exhibits characteristics of a standing waveform. FIG. 2 is a graph showing the varying amplitudes of ultrasonic vibration 120, 130 along the transducer 100 of the prior art when ultrasonic energy is generated at frequencies of 97 kHz and 138 kHz respectively. In order to deliver a maximum amplitude of ultrasonic vibration for the bonding process, the tip of the capillary 110 is preferably disposed at anti-nodal positions 122, 132 of the standing waveforms. This is at the tip 108 of the horn 102 where the amplitude of ultrasonic vibration 120, 130 is maximum.
A holding portion 114 of the transducer 100 where the transducer 100 is clamped by a bond head is preferably disposed at nodal positions 124 of the standing waveforms where there is a minimum amplitude of ultrasonic vibration. At such locations, the ultrasonic vibration is ideally zero, so that the transmission of ultrasonic energy out of the horn 102 through the holding portion 114 is minimized, and hence ultrasonic energy losses through the transducer holder of the bond head supporting the transducer 100 are minimal.
However, the transducer 100 is in continual motion during the bonding operation when the bond head relocates the capillary 110 for performing bond placement at different bond pad locations. Therefore, the transducer 100 is subjected to low cycle fluctuating forces in transverse directions with respect to the longitudinal axis of the transducer 100. This will adversely affect its rigidity, and in particular its dynamic rigidity, and hence the bonding quality.
Various transducer mounting methods have been implemented in the prior art to minimize the loss of ultrasonic energy transmitted through the mounting attachment of the transducer 100. In a single nodal mount approach illustrated in U.S. Pat. No. 5,603,445 entitled “Ultrasonic Wire Bonder and Transducer Improvements”, the transducer is designed to generate a standing wave comprising one wavelength and a mounting bracket is positioned at a nodal position which is at a distance of ¾ wavelength from an anti-nodal position of the transducer at which a capillary is disposed.
It should be understood that by having the holding portion 114 of the mounting bracket disposed at the nodal position 124 of the ultrasonic waveform, the connection between the holding portion 114 and the horn 102 should be as thin as possible so that the loss of ultrasonic energy through the mounting bracket is minimized. On the other hand, a very thin single nodal holding portion 114 on the transducer 100 of the prior art is generally at the cost of its dynamic rigidity.
Alternatively, a dual nodal mount approach illustrated in U.S. Pat. No. 6,719,183 entitled “Transducer and a Bonding Apparatus Using the Same” attempts to overcome some of the above-mentioned problems by providing at least two holding portions on the mounting bracket such that locations of the multiple holding portions correspond to the nodes of ultrasonic vibration of the transducer.
Although this approach of holding the transducer 100 at two positions may produce a more rigidly mounted transducer, one of the constraints in this dual nodal mount approach is that the transducer is only operable in a single ultrasonic frequency mode. Contrast this to FIG. 2, where the graph shows different variations in the amplitudes of ultrasonic vibration 120, 130 along the transducer 100 of the prior art when ultrasonic energy is generated at frequencies of 97 kHz and 138 kHz respectively. Due to the differences in the wavelengths for different operating frequencies, ultrasonic energy generated at an operating frequency of 97 kHz is such that the arrangement of the nodes 124 and the anti-nodes 122 on the standing waveform 120 may substantially differ from the arrangement of the nodes 124 and the anti-nodes 132 on the standing waveform 130 for ultrasonic energy generated at an operating frequency of 138 kHz. Therefore, if the holding portions 114 are rigidly fixed at two different nodal positions at a particular operating frequency, the nodal positions of a second operating frequency may not coincide with that of the aforesaid first operating frequency at the same location of the holding portions 114.
As such, a transducer which is clamped at nodal positions corresponding to the first operating frequency will be operating less effectively when driven with the second operating frequency as ultrasonic energy is lost through transmission to the bond head because the holding portions are no longer located at the nodal positions. Hence, by fixing the holding portions 114 at two different nodal positions at a particular operating frequency, the dual nodal mount approach constrains the transducer to be operable in only a single ultrasonic frequency. It would be desirable to permit different operating frequencies to be used with the transducer for different bonding requirements.
Therefore, it would be advantageous to avoid some of the aforesaid disadvantages of the prior art by having a transducer that is operable at two or more operating frequencies, and yet has high dynamic rigidity.